[cedias]

It actually seems like an overhead but in reality it's designed to help you maintain your software easier: Using interfaces makes your software functionnally dependant meaning that it relies on "theorical functions". On the contrary using directly some library makes your software structurally dependant to one's library. This can be a massive issue if the library you're using is not maintained. If you wanted to switch to another one unless you were using a well defined interface that the other library can implement. You would just have to change a few lines of code vs changing a whole lot of calls to one's lib.

IMO, this is one of the major issue of javascript.